PRELIMINARY LUMINESCENCE DATES FROM THE PASO ROBLES FORMATION IN THE SANTA MARIA BASIN SUGGEST A LATE QUATERNARY UPLIFT EVENT IN THE SOUTHERNMOST COAST RANGES

The western Transverse Ranges and the southern Coast Ranges of California differ in many ways structurally. Faults and folds in the southern Coast Ranges trend northwest with slow uplift rates of about 1mm/yr, while structures in the western Transverse Ranges trend east/west with faster uplift rates of about 1-7mm/yr. The Santa Maria Basin is considered the ‘transition zone’ between the two and is a key area for understanding the tectonic history of California. To better understand the differences in timing of rock uplift between the western Transverse Ranges and southern Coast Ranges, we used the Paso Robles Formation, a Pleistocene sedimentary deposit, as a marker for uplift of the surrounding ranges from which it derived. The Paso Robles Formation is composed of alluvial sediment that was derived from the San Rafael and Santa Ynez Mountains. Using previously published geologic maps, we selected key locations to date the Paso Robles Formation in the Santa Maria Basin, aiming to constrain a depositional period. We dated 8 samples from 5 sites at a range of stratigraphic levels of the Paso Robles using luminescence dating techniques. These dates range from 110 ka to 170 ka. We present these preliminary dates as the first numeric ages of the Paso Robles Formation in this region, and possibly the most recent uplift event in the southern Coast Ranges. Our new ages suggest that the segment of the southern Coast Ranges in this study area were lifted more recently than previously thought. This has significant implications for understanding the geological history and processes of the southern Coast Ranges.